Abstract: In order to understand the effects of different nitrogen forms and magnesium concentrations on the photosynthetic fluorescence characteristics and growth of flue-cured tobacco at early stages, using Cuibi No. 1 flue-cured tobacco as the material, three nitrogen treatments and two magnesium levels were set under low temperature and low light by a hydroponics test method, and the photosynthetic fluorescence characteristics and growth differences of different treatments were analyzed. The results showed that biomass, maximum net photosynthetic rate (P_max), apparent quantum efficiency (AQY) and the overall functional activity index (PI_total) of the electron transport chain between PSII and PSI systems were the highest in the treatment with 50% ammonium and 50% nitrate under the condition of magnesium application. The biomass of the treatment with 50% ammonium and 50% nitrate increased by 21.7% compared with the treatment with pure nitrate nitrogen, and increased by 96.0% compared with the treatment with pure ammonium nitrogen under the condition of magnesium application. P_max and AQY of the pure nitrate nitrogen treatment were higher than those of the treatment with 50% ammonium and 50% nitrate and significantly higher than those of the pure ammonium nitrogen treatment under the condition of no magnesium. The addition of magnesium to the treatment of pure nitrate and treatment with 50% ammonium and 50% nitrate significantly improved all the indexes, but had no significant effect on the treatment of pure ammonia. The results showed that under the conditions of low temperature and weak light, the treatment with 50% ammonium and 50% nitrate and magnesium application can significantly improve chloroplast pigment content, electron transport efficiency, enhance photosynthetic capacity, promote biomass accumulation, and is more conducive to the early growth and development of flue-cured tobacco.